Method And Device For Stabilizing The State Of Polarization Of A Polarization Multiplexed Optical Radiation

ABSTRACT

A device and method for stabilizing the polarization of polarization multiplexed optical radiation includes an identified channel which is provided with a pilot signal. The device and method are based on providing to the polarization multiplexed optical radiation a controllable polarization transformation; measuring the optical power of a polarized portion of the identified channel downstream the polarization transformation; controlling, responsively to the optical power, the controllable polarization transformation so that the identified channel downstream the polarization transformation has a predefined polarization azimuth; providing to the optical radiation downstream the polarization transformation a further controllable polarization transformation; measuring the optical power of a polarized portion of the identified channel downstream the further controllable polarization transformation; and controlling, responsively to the optical power, the further controllable polarization transformation so that the identified channel downstream the further controllable polarization transformation has a predefined state of polarization.

The invention relates to polarization stabilization, more especially tomethods and devices for stabilizing with a high accuracy thepolarization state of an optical radiation of arbitrary, possibly timevariant, polarization.

A polarization stabilizer is a device that transforms an input opticalbeam having an input state of polarization (SOP) into an output opticalbeam with a predetermined SOP and an optical power, both not dependenton the input SOP. In general, a defined SOP is determined by twoparameters: the ellipticity and the polarization azimuth. Such a deviceis useful, for example, in coherent optical receivers for matching theSOP between the signal and the local oscillator, in fiber opticinterferometric sensors, in compensation of polarization mode dispersionof the transmission line and in optical systems with polarizationsensitive components. An important requirement is the endlessness incontrol, meaning that the stabilizer must compensate in a continuous wayfor the variations of input SOP.

In polarization division multiplexing (PoIDM) transmission at least twooptical channels, each comprising an optical carrier, are launchedorthogonally polarized in the optical transmission medium, such as forexample an optical transmission fiber. In a typical solution for PoIDMtransmission, the two optical carriers of the at least two orthogonallypolarized optical channels are spectrally closely spaced, such as forexample within an optical spectrum spacing of 50 GHz or within a 25 GHzspacing. In a preferred configuration, the two carriers, and hence thetwo channels, have substantially the same optical wavelength. Typically,while the reciprocal orthogonality of the state of polarization issubstantially preserved along the propagation into the transmittingmedium, the absolute SOPs of the two channels randomly fluctuate at agiven position along the line, such as for example at the receiversection.

In PoIDM, a problem arises at the receiving section, or whenever the twoorthogonally polarized channels need to be polarization demultiplexed.In general, the polarization demultiplexer is typically a polarizationbeam splitter, which is apt to split two orthogonal SOPs. In case of anerror in polarization locking, a misalignment occurs between the SOPs ofthe two channels and the orthogonal SOPs divided by the polarizationdemultiplexer. In this case a cross-talk is generated due to aninterference between a desired channel and the small portion of theother non-extinguished channel, which severely degrades the quality ofthe received signal. For example, in PoIDM systems having the individualchannels intensity modulated with non-return-to-zero format and directlydetected (IM-DD), the penalty to the bit-error-rate becomes about 1 dBfor cross-talk of about 20 dB. This means that in case the intensity ofthe non-extinguished channel is greater than or equal to about 1% of theintensity of the demultiplexed channel, the cross-talk becomes aconcern.

Accordingly, in PoIDM systems a highly accurate polarizationstabilization of the SOPs of the two polarization multiplexed channelsis needed before polarization demultiplexing. The cross-talk afterpolarization demultiplexing is related to the accuracy of polarizationstabilization. In case of a single optical channel, the accuracy of apolarization stabilizer in terms of optical power may be expressedthrough a parameter, called uniformity error, defined according to$\begin{matrix}{{U = \frac{I_{\max} - I_{\min}}{I_{\max} + I_{\min}}},} & (1)\end{matrix}$wherein I_(max) and I_(min) are the actual maximum and minimum opticalintensities, in locked operation, of the polarization-stabilized outputradiation of the channel when varying the input SOP. In general, thesmaller is the uniformity error, the smaller results the cross-talkafter demultiplexing. For example, under simplified conditions, auniformity error of about 1% gives rise to a cross-talk of about 2%.

The patent application US2004/0016874 discloses (see FIG. 4 thereof) anautomatic polarization controller for a polarization multiplexed opticalpulse train including at least one dither modulation signal, thepolarization controller including a polarization transformer of anytype. A polarization selective element receives the transformedpolarization multiplexed optical pulse train and passes a polarizedoptical pulse train including the dither modulation signal. A detectorreceives the polarized optical pulse train including the dithermodulation signal and generates a signal that is proportional to theamplitude of the dither modulation signal. A feedback control unitgenerates a control signal that is coupled to the control input of thepolarization transformer.

The patent application US2002/0191265 discloses (see FIG. 3 thereof) atwo-stage electro-optic polarization transformer for transforming thepolarization states of an orthogonally polarized polarizationmultiplexed optical signal comprising a first and a second component. Anoptical feedback signal is extracted from the output of the second stagepolarization transformer. In one embodiment, the first and the secondcomponents of the polarization multiplexed optical signal are identifiedwith different dither frequencies. A mixer generates a signal that has afrequency that identifies the component of the polarization multiplexedoptical signal.

The Applicant has noted that the polarization controllers disclosed inboth the documents above directly detect the SOP of the opticalradiation only downstream the polarization transformer itself (onlydownstream the second stage in the second document) and send a singlefeed-back signal to the feedback control unit. The methods disclosedthus require complicate elaboration of the electrical feedback signaland complicate control algorithm, without adding in precision to thepolarization stabilization.

WO03/014811 patent discloses an endless polarization stabilizer based ona two-stage configuration wherein the two stages are controlledindependently by an endless polarization stabilizing method based on asimple feedback control algorithm. Each stage comprises a pair ofbirefringent components that each have fixed eigenaxes and variablephase retardation. The two birefringent components are variableretarders with finite birefringence range and respective eigenaxesoriented at approximately ±45 degrees relative to each other. Theendlessness is obtained by commuting the phase retardation of oneretarder, when the retardation of the other retarder reaches a rangelimit.

The Applicant has found that none of the known solutions forpolarization stabilization is at the same time suitable for working witha polarization multiplexed optical radiation, accurate enough to meetthe specifications in the context of PoIDM demultiplexing and simpleenough to be practically feasible and operable.

The Applicant has thus faced the problem of providing a simple, feasibleand highly accurate method and device to stabilize the state ofpolarization of a polarization multiplexed optical radiation having anarbitrary SOP to a predetermined output SOP, while keeping the outputoptical power not dependent on the input SOP. In particular, theApplicant has sought an accuracy suitable for polarizationdemultiplexing applications in PoIDM systems; for example the uniformityerror is preferably less than or equal to 1%.

The Applicant has found that in the context of PoIDM systems, in orderto achieve highly accurate stabilization of the SOP, it is advantageousto achieve first an highly accurate stabilization of one out of the twopolarization parameters (ellipticity and azimuth) and after that ahighly accurate full stabilization of the SOP.

The Applicant has found that a method and a device based on two stageseach comprising a respective variable birefringent element,independently controlled by a respective simple and effective feedbackcontrol algorithm, wherein the SOP of the optical radiation outputtingfrom the first stage is directly detected through a monitoring systemwhich is sensitive to a pilot signal contained in the optical radiation,provides polarization stabilization of a polarization multiplexedoptical radiation with the degree of accuracy needed for polarizationdemultiplexing in PoIDM systems and the feasibility and operabilityneeded for industrial application. The Applicant has sought inparticular a method and device for endlessly stabilize the polarizationof a polarization multiplexed optical radiation.

In some polarization control schemes based on finite range components,in order to achieve an endless control, it has been proposed a resetprocedure when a component reaches its range limit so that the outputSOP does not change during the reset. Generally, reset procedures can beproblematic in that they are often associated with complex controlalgorithms designed to avoid loss of feedback control during the reset.

The Applicant believes that a polarization stabilizing method and deviceaccording to the above, wherein each stage comprises two variableretarders, in combination with a simple and effective control algorithm,which avoids reset procedure and is based on the commutation of thefirst retarder when the second reaches a retardation range limit,provides the speed, the degree of accuracy and the feasibility neededfor polarization demultiplexing in PoIDM systems.

The Applicant has found that a polarization stabilizer device whereinthe variable retarders are variable rotators and each stage alsocomprises a fixed quarter-wave plate between them, adds further accuracyand feasibility to polarization stabilization of a polarizationmultiplexed radiation.

In a first aspect, the present invention relates to a method forstabilizing the state of polarization of a polarization multiplexedoptical radiation comprising an identified channel which is providedwith a pilot signal, the method comprising: providing to thepolarization multiplexed optical radiation a first controllablepolarization transformation to generate a first transformed opticalradiation; measuring a first optical power of a first polarized portionof said identified channel of the first transformed optical radiation;controlling, responsively to said first optical power, the firstcontrollable polarization transformation so that the identified channelof the first transformed optical radiation has a predefined polarizationazimuth; providing to the first transformed optical radiation a secondcontrollable polarization transformation to generate a secondtransformed optical radiation; measuring a second optical power of asecond polarized portion of said identified channel of the secondtransformed optical radiation; controlling, responsively to said secondoptical power, the second controllable polarization transformation sothat the identified channel of the second transformed optical radiationhas a predefined state of polarization.

Preferably, said first polarized portion of said identified channel ofthe first transformed optical radiation has the polarization azimuth at±45° with respect to said predefined polarization azimuth.Advantageously, said second polarized portion of said identified channelof the second transformed optical radiation has the state ofpolarization parallel or perpendicular to said predefined state ofpolarization.

Preferably, the first optical power of the first polarized portion ofsaid identified channel of the first transformed optical radiation ismeasured through measuring a modulation amplitude of said pilot signal.More preferably, said modulation amplitude is measured after extractinga power fraction from the first transformed optical radiation,polarizing said power fraction to generate a polarized power fraction,detecting said polarized power fraction and pass-band filtering saiddetected polarized power fraction to obtain said modulation amplitude.

Advantageously, also the second optical power of the second polarizedportion of said identified channel of the second transformed opticalradiation is measured through measuring a modulation amplitude of saidpilot signal.

In order to measure said modulation amplitude, it is preferable toextract a power fraction from the second transformed optical radiation,polarize said power fraction to generate a polarized power fraction,detect said polarized power fraction and pass-band filter said detectedpolarized power fraction to obtain said modulation amplitude.

The method according to the present invention may further comprisemeasuring a third optical power of a third polarized portion of saididentified channel of the first transformed optical radiation.Preferably, said third polarized portion of said identified channel ofthe first transformed optical radiation has the polarization azimuthorthogonal to the polarization azimuth of said first polarized portion.

In a further preferred embodiment, the first controllable polarizationtransformation is endlessly varying. Also the second controllablepolarization transformation may be endlessly varying.

In a second aspect of the present invention, it is disclosed a method ofdemultiplexing a polarization multiplexed optical radiation, the methodcomprising any method described above and further comprising separatingthe identified channel in the second transformed optical radiation froma further channel orthogonally polarized to the identified channel.

In a third aspect, the present invention is a method of transmitting apolarization multiplexed optical signal, the method comprising:providing a pilot signal to an optical channel to generate an identifiedchannel; polarization multiplexing the identified channel with a furtherchannel at a first location to generate a polarization multiplexedoptical radiation; propagating said polarization multiplexed opticalradiation at a second location different from the first location;stabilizing the state of polarization of the polarization multiplexedoptical radiation at the second location according to any of the methoddescribed above to generate a polarization stabilized optical radiation;separating the identified channel of the polarization stabilized opticalradiation from the further channel and detecting at least one of saididentified and further channel.

In a fourth aspect, the invention relates to a device for stabilizingthe state of polarization of a polarization multiplexed opticalradiation comprising an identified channel which is provided with apilot signal, the device comprising a first polarization transformercomprising a first birefringent element operable to provide a firstvariable polarization transformation to the polarization multiplexedoptical radiation; a first monitoring system responsive to said pilotsignal and apt to measure the optical power of a first polarized portionof the identified channel downstream the first polarization transformer;a controller apt to control, responsively to the optical power of saidfirst polarized portion, said first variable polarization transformationso as to maintain the polarization azimuth of the identified channeldownstream the first polarization transformer at a predefined azimuth; asecond polarization transformer positioned downstream the firstpolarization transformer and comprising a second birefringent elementoperable to provide a second variable polarization transformation to thepolarization multiplexed optical radiation; a second monitoring systemresponsive to said pilot signal and apt to measure the optical power ofa second polarized portion of the identified channel downstream thesecond polarization transformer; and wherein the controller is furtherapt to control, responsively to the optical power of said secondpolarized portion, said second variable polarization transformation soas to maintain the state of polarization of the identified channeldownstream the second polarization transformer at a defined state ofpolarization.

Preferably, the first monitoring system is further apt to measure theoptical power of a further polarized portion of the identified channeldownstream the first polarization transformer, wherein said furtherpolarized portion is orthogonal to the first polarized portion.

The first polarization transformer may further comprise a thirdbirefringent element operable to provide a third variable polarizationtransformation to the polarization multiplexed optical radiation. Inthis case, it is preferable that the controller is configured to switchthe third variable polarization transformation between first and secondvalues when the first variable polarization transformation reaches apredefined threshold value, in order to provide a reset-free endlesscontrol.

Preferably, each of the first birefringent element and the thirdbirefringent element comprises a respective variable rotator and thefirst polarization transformer further comprises a quarter-wave plateoptically interposed between the first and the third birefringentelement.

The second polarization transformer may further comprise a fourthbirefringent element operable to provide a fourth variable polarizationtransformation to the polarization multiplexed optical radiation. Inthis case, the controller is configured to switch the fourth variablepolarization transformation between third and fourth values when thesecond variable polarization transformation reaches a predefinedthreshold value in order to provide a reset-free endless control.

Advantageously, the first monitoring system is configured to measure amodulation amplitude of said pilot signal so as to measure said opticalpower of said first polarized portion. Preferably, the first monitoringsystem comprises a splitter for extracting a power portion of saidpolarization multiplexed optical radiation, a polarization splitter forextracting a polarized portion of said power portion, a photodiode forgenerating a signal from said polarized portion of said power portionand a demodulator for band-pass filtering said signal to obtain saidmodulation amplitude of said pilot signal.

In a fifth aspect, the invention relates to an optical polarizationdemultiplexer comprising the polarization stabilizing device describedabove and a polarization division demultiplexer, such as e.g. apolarization beam splitter, located downstream the polarizationstabilizing device and oriented parallel or perpendicular to saiddefined state of polarization.

In a sixth aspect, the invention relates to a polarization divisionmultiplexing system comprising a polarization transmitter apt to combinea first and a second optical channel having orthogonal polarization,wherein the first channel comprises a pilot signal; a transmission lineapt to transmit said combined first and second optical channel; and anoptical polarization demultiplexer describe above, optically coupled tosaid transmission line, and apt to separate said first and secondoptical channel.

For a better understanding of the invention and to show how the same maybe carried into effect reference is now made by way of example to theaccompanying drawings.

FIG. 1. Schematic drawing of a polarization division multiplexingoptical system according to one aspect of the present invention.

FIG. 2. Schematic drawing of a base architecture of the polarizationstabilizer device according to the present invention.

FIG. 2 a. Schematic drawing of an alternative configuration of the firststage of the polarization stabilizer device of FIG. 2.

FIG. 3. Schematic drawing of a first exemplary embodiment of thepolarization stabilizer device of FIG. 2.

FIG. 4. Poincaré sphere representation of a polarization stabilizerdevice according to the first embodiment of the present invention.

FIG. 5 a. Four exemplary points on the Poincaré sphere representing fourexemplary input SOPs to the polarization stabilizer of the presentinvention.

FIGS. 5 b-5 e. SOP transformations on the Poincaré sphere generated bythe first embodiment of the present invention polarization stabilizercorresponding to the four input SOPs of FIG. 5 a

FIG. 6 a-6 c. SOP transformations on the Poincaré sphere generated bythe first embodiment of the present invention polarization stabilizer.

FIG. 7. Diagram of bit error rate (BER) versus power at the receiver ofan optical system employing the present invention polarizationstabilizer device with different pilot tone modulation index.

FIG. 8. Diagram of bit error rate (BER) versus power at the receiver ofan optical system with and without polarization stabilization accordingto the present invention.

FIG. 9. Schematic drawing of a second exemplary embodiment of thepolarization stabilizer device of FIG. 2.

FIG. 10. Schematic drawing of a third exemplary embodiment of thepolarization stabilizer device of FIG. 2.

FIG. 1 schematically shows a polarization division multiplexing system 1in accordance with one aspect of the present invention.

A transmitter section TX is apt to encode data information into apolarization multiplexed optical radiation comprising two opticalchannels orthogonally polarized. The transmitter section TX may includeoptical sources (e.g. lasers), modulators (e.g. electro-opticmodulators), wavelength multiplexers, polarization multiplexers, opticalboosters, etc. One of the two channels, hereinafter referred to as theidentified channel, is provided with a pilot signal which may serve touniquely identify said channel. Optionally, the other of the twochannels may also be provided with a second pilot signal uniquelyidentifying it.

The pilot signal may be a superimposed modulation such as for example anamplitude or intensity modulation, a phase modulation, an opticalfrequency modulation or a polarization modulation, or it may be anidentifying clock, for example an identifying bit-clock. Thesuperimposed modulation may follow any given waveform, such as forexample an harmonic wave (hereinafter called pilot tone in case ofintensity modulation) or a square wave (usually called dither). Thefrequency of modulation of the superimposed modulation should be lowenough with respect to the data modulation rate (bit-rate) in order notto degrade the transmission quality. For example, in case of a bit-rateof 622 Mb/s or greater, it is advantageous to set the pilot signalfrequency less than or equal to 10 MHz. On the other end, the frequencyof modulation of the pilot signal should be high enough to differ fromthe continuous (zero frequency) spectral component. A possible range forthe pilot signal frequency is from 1 kHz to 10 MHz, including the endsof range.

The two channels are launched into an optical transmission line 2 withmutually orthogonal state of polarization. The optical transmission line2 may include for example an optical cable comprising optical fibers.Optical line amplifiers LA, such as for example EDFAs, may bedistributed along the optical transmission line 2. Also, one or moreoptical processing units OPU may be placed along the line 2 in order toperform operations on the optical signal such as routing, regeneration,add and/or drop, switching and the like. A receiver section RX is placedat the end of the transmission line 2 or whenever the optical signalneeds to be received (e.g. at the OPU), in order to convert the opticalsignal into an electrical signal. It may comprises opticalpre-amplifiers, optical filters, photodetectors, electrical filters,etc.

A polarization stabilizer device 100 according to the present inventionis placed upstream the receiver section RX in order to stabilize the SOPof the polarization multiplexed optical radiation to a defined SOPbefore inputting the receiving section RX. In other words, the SOP ofone of the two optical channels inputting the polarization stabilizerdevice 100 is converted to a defined SOP and consequently the SOP of theother of the two optical channels is uniquely stabilized to a SOPorthogonal to the defined SOP. Throughout the present description,reference will be made to the SOP of the identified channel, being theSOP of the other optical channel uniquely determined.

In case a wavelength division multiplexing (WDM) technique is used incombination with PoIDM in the optical transmission system 1, each WDMcarrier wavelength comprises two orthogonally polarized channels whereinone channel of each couple is identified by a pilot signal. In thiscase, a wavelength demultiplexer D-MUX may be placed upstream thepolarization stabilizer 100 in order to separate, at least partially,the different optical wavelengths.

Advantageously, a polarization selective element PS, for example apolarization division demultiplexer such as a polarization beam splitterhaving its azimuth oriented parallel or perpendicular to the definedSOP, may be placed at the output end of the polarization stabilizerdevice 100 in order to separate the two polarization multiplexedchannels. The polarization selective element PS may be integrated eitherwithin the polarization stabilizer device 100 or within the receiversection RX.

In case the two orthogonally polarized optical channels are closelyspaced in the optical spectrum without overlapping(polarization-interleaved WDM), it is preferable to superimpose a pilotsignal to each WDM channel. For example, odd channels have a first pilotsignal and even channels have a second pilot signal (e.g. havingfrequency different from the first one). In this case, the wavelengthdemultiplexer D-MUX placed upstream the polarization stabilizer device100 passes the desired WDM channel and one or more undesired adjacentoptical channels. The desired WDM channel has a SOP orthogonal to theSOP of the adjacent channels. In polarization-interleaved WDM thepolarization selective element PS is advantageously a linear polarizer.The polarization stabilizer device 100 thus acts to align the SOP of thedesired WDM channel to the polarizer by making use of the pilot signalof the desired channel. The residual portion of the adjacent WDMchannels are thus filtered out by the polarizer.

FIG. 2 is a schematic representation of a base architecture of thepolarization stabilizer device 100 according to the present invention.

The device 100 comprises a first and a second stage 200 and 300.

The device 100 has a principal beam path ‘x’ along which a polarizationmultiplexed optical radiation is received as an input optical radiationof arbitrary state of polarization of the identified channel (labeledSOP_(IN) in the figure); the radiation then traverses the first stage200 and outputs the first stage 200 with a SOP (labeled SOP_(INT))having the polarization azimuth at a predefined value. Conventionally,the polarization azimuth will range from −90° to +90° modulus 180°. Forthe purpose of the present invention, a predefined value of thepolarization azimuth means a couple of angular values differing of 90°.Examples of predefined polarization azimuth are (−30°, +60°) or (0°,+90°) or (−45°, +45°). It is noted that also the polarization azimuth ofthe other orthogonally polarized channel is at the same predefinedvalue. The polarization multiplexed optical radiation then traverses thesecond stage 300 and is emitted from the device 100 as an opticalradiation having a stabilized defined SOP of the identified channel(labeled SOP_(OUT)) and an optical power not depending on the input SOP.Without loss of generality, the defined SOP may be the linear verticalSOP having defined vertical azimuth and defined zero ellipticity.

The first stage 200 comprises a polarization transformer PT1 which isapt to give to the optical radiation propagating through it a firstcontrollable polarization transformation. The polarization transformerPT1 may comprise a birefringent element BE1 or a combination of singlebirefringent elements including BE1.

The first stage 200 also comprises a monitoring system MS1 which isresponsive to the pilot signal of the identified channel and is apt tomeasure uniquely the optical power of a polarized portion of theidentified channel outputting from the polarization transformer PT1.Throughout the present description, the term “polarized portion” or“polarized component” means the projected component of the opticalradiation along a given SOP. For sake of clarity, in case of deviationof the optical radiation, for example a reflection by a beam splitter,the reference system for the SOP is accordingly transported.

The first stage 200 also comprises a controller CTRL1 (e.g. anelectronic controller or a computer) which is apt to control the firstcontrollable polarization transformation, given by the polarizationtransformer PT1 through e.g. the birefringent element BE1, in responseto the optical power of the polarized portion of the identified channelmeasured by the monitoring system MS1, so that the azimuth of the SOP ofthe identified channel outputting from the polarization transformer PT1remains at a target predefined value. The controller CTRL1 is connectedto the monitor system MS1 and to the polarization transformer PT1. Thesecond stage 300 comprises a polarization transformer PT2 which is aptto give to the optical radiation propagating through it a secondcontrollable polarization transformation. The polarization transformerPT2 may comprise a birefringent element BE2 or a combination ofbirefringent elements including BE2. The second stage 300 also comprisesa monitoring system MS2 which is responsive to the pilot signal of theidentified channel and is apt to measure uniquely the optical power of apolarized portion of the identified channel outputting from thepolarization transformer PT2.

The second stage 300 also comprises a controller CTRL2 (e.g. anelectronic controller or a computer) which is apt to control the secondcontrollable polarization transformation, given by the polarizationtransformer PT2 through e.g. the birefringent element BE2, in responseto the optical power of the polarized portion of the identified channelmeasured by the monitoring system MS2, so that the SOP of the identifiedchannel outputting from the polarization transformer PT2 remains at adefined SOP. The controller CTRL2 is connected to the monitor system MS2and to the polarization transformer PT2.

Although, for the sake of clarity, two separate controllers CTRL1 andCTRL2 have been described and represented in FIG. 2, it can beappreciated that a single controller can advantageously be employed,connected in input to the monitoring systems MS1 and MS2 and in outputto the polarization transformers PT1, PT2. Advantageously, apolarization division demultiplexer PDD, which is a particularembodiment of the polarization selective element PS of FIG. 1, may beplaced along the main beam path ‘x’ downstream the second stage 300 toseparate the polarization multiplexed optical channels. For example, apolarizing beam splitter oriented with its azimuth extending parallel orperpendicular to the defined output azimuth.

The polarization transformers PT1 and PT2 are placed on the principalbeam path ‘x’. The birefringent element BE1 and BE2 may be any kind ofbirefringent element apt to give a variable polarization transformation,such as for example a variable retarder having fixed eigenstates andvariable phase retardation, or birefringent element having fixed phaseretardation and variable eigenstates (e.g. rotating axes), or variableeigenstates and variable phase retardation.

In general any physical mechanism producing a birefringence can beexploited to realize the birefringent elements used in the polarizationstabilizer device 100 of the present invention. For example, they may bebased on the magneto-optic effect (e.g. Faraday rotator), theelectro-optic effect (such as the nematic liquid-crystal retarders orthe electro-optic crystals belonging to the symmetry point group of thezincblende such as zinc sulfide (ZnS) with its ternary or higher ordercompounds (e.g. ZnSSe); cadmium telluride (CdTe) with its ternary orhigher order compounds (e.g. CdZnTe); gallium arsenide (GaAs) with itsternary or higher order compounds (e.g. AlGaAs, InGaAsP); and the like)or the elasto-optic effect (such as squeezers).

The monitor systems MS1 and MS2 are associated to the principal beampath ‘x’. They are designed to be responsive to the pilot signal.Accordingly, they are apt to identify the identified channel through thepilot signal and to measure only the optical power of the identifiedchannel.

In FIG. 2, it is shown an exemplary embodiment of the monitoring systemsMS1 and MS2 apt to be used in connection with a superimposed amplitudeor intensity modulation as the pilot signal of the identified channel.

Accordingly, the monitoring systems MS1 and MS2 of the first and secondstage 200 and 300 may comprise a polarization insensitive beam-splitter,respectively BS1 and BS2, arranged in the beam path ‘x’ downstream therespective polarization transformer PT1 and PT2. BS1 and BS2 are apt toextract a small fraction of the optical radiation outputting from therespective polarization transformer PT1 and PT2. For minimum losses, theextracted portion of the radiation should be vanishingly small. However,in practice, the diverted portion needs to be large enough to provide areasonable signal-to-noise ratio for subsequent processing associatedwith the control loop. A diverted power fraction of between 1 and 10%may be typical. It will be appreciated that other optical components canprovide the same function of extracting a small fraction of the beam,for example an optical fiber coupler.

The monitor system MS1 of the first stage 200 may comprise a polarizingbeam splitter PBS optically connected to the beam splitter BS1, as shownin FIG. 2. The PBS is apt to receive the optical radiation extracted bythe beam splitter BS1. The azimuth of the PBS is approximately at ±45°with respect to the predefined azimuth. For example, at a predefinedazimuth of (−30°, +60°) corresponds a PBS azimuth of +15° or −75°. Inother words, the PBS is apt to separate a linearly polarized portion ofthe extracted optical beam having an azimuth at +45° to the predefinedazimuth from a linearly polarized portion of the optical radiationhaving an azimuth at −45° to the defined azimuth.

Throughout the present description, a polarization beam splitter PBS isfunctionally equivalent, and interchangeable, to a polarizationinsensitive beam splitter followed by two orthogonally oriented linearpolarizers, one for each output of the polarization insensitive beamsplitter. Optical fiber or optical waveguides components can also beused to provide the same function.

It is noted that at each output of the PBS, the polarized portions ofboth the polarization multiplexed optical channels are present andoverlapping.

A first and a second photodiode PD1 and PD2 may be optically connectedto the polarizing beam splitter PBS, one for each output of the PBS.They are apt to detect the two polarized components of the opticalradiation outputting respectively from the two outputs of the PBS and togenerate respective signals responsive of the optical power of these twopolarized components.

In particular applications, for example when the power of the inputoptical beam is known and can be held constant, either photodiode PD1 orphotodiode PD2 may be omitted. In this case, the polarizing beamsplitter PBS may be replaced by a fixed linear polarizer oriented eitherat +45° or −45° to the predefined azimuth.

The monitor system MS2 of the second stage 300 may comprise a linearpolarizer P2, preferably fixed, optically connected at the reflectedoutput of the beam splitter BS2, as shown in FIG. 2. The polarizer P2 isapt to receive the optical radiation extracted by the beam splitter BS2.The azimuth of the P2 is approximately parallel or perpendicular withrespect to the defined output SOP. In other words, the polarizer P2 isapt to pass a linearly polarized portion of the extracted optical beamhaving a SOP parallel or perpendicular to the defined SOP.

A photodiode PD3 may be connected to the output end of the polarizer P2and is apt to measure the optical power of the extracted polarizedportion and generate a signal responsive of this power.

A first, a second and a third demodulator DM1, DM2 and DM3 may beconnected to the first, second and third photodiode PD1, PD2 and PD3,respectively. The first, second and third demodulator DM1, DM2 and DM3are apt to receive respective signal from first, second and thirdphotodiode PD1, PD2 and PD3 and to respond to the pilot signal. Forexample, in case a pilot tone (sinusoidal amplitude modulation) is usedfor the identified channel, each demodulator executes a pass-bandfiltering of the electrical signal generated by the correspondingphotodiode, around the pilot tone frequency. Such a filtered signal,neglecting the noise terms, can be expressed as a sinusoid at a pilottone frequency f_(PT) with an amplitude of modulation A_(i)(t) accordingto:s _(i)(t)=A _(i)(t)sin(2πf _(PT) t)  (1)where i=1, 2, 3 refer respectively to the first, second and thirddemodulator DM1, DM2 and DM3. The i-th pilot tone amplitude A_(i)(t) isdirectly proportional to the optical intensity of solely the polarizedportion of the identified channel incident on the corresponding i-thphotodiode. The action of the demodulators is to measure these pilottone amplitudes A_(i)(t), carrying the information about the SOP of theidentified channel and used by the controllers CTRL1 and CTRL2 for theSOP stabilization. Such a demodulator DM1, DM2 or DM3 can be realized byusing any electrical scheme among those well known in radio engineeringfor detecting an amplitude modulation of a carrier. For example thedemodulator may be based on envelope detection or coherent detectionschemes.

The first, second and third demodulator DM1, DM2 and DM3 may generaterespective output signals V₁, V₂ and V₃, indicative of the respectivepilot tone amplitudes A₁, A₂ and A₃, which in turn are indicative of theoptical powers of the respective polarized portions of the identifiedchannel. It will be appreciated that these signals may be in electronicform, with the photodiodes being optoelectronic converters and thedemodulators being electronic circuits. However, it will also beappreciated that these processing elements could be embodied withall-optical components of the same functionality. This may be desirablefor stabilizing extremely high frequency polarization instabilitieswhere all-optical power sensing and control processing could beperformed. In addition, the signals V₁, V₂ and V₃ may also be radiosignals. It will be also appreciated that demodulation of the pilotsignal may be performed directly by the photodiodes PD1, PD2 and PD3.

In those applications, described above, wherein either photodiode PD1 orphotodiode PD2 may be omitted, also the respective demodulator DM1 orDM2 and the respective signal V₁ or V₂ may be omitted.

The function of the first stage 200 of the device 100 is to transformany input SOP of the identified channel into an elliptical output SOP(SOP_(INT)) with major axis (said polarization azimuth) at a predefinedazimuth.

In operation, the input polarization multiplexed optical radiationhaving an identified channel traverses the polarization transformer PT1.The polarization transformer gives to the optical radiation a variablecontrollable polarization transformation, such that the SOP of theidentified channel is transformed from SOP_(IN) to SOP_(INT), outputtingfrom the polarization transformer PT1, wherein SOP_(INT) has an azimuthat a predefined value.

A feedback control loop is designed to lock the polarization azimuth ofthe SOP (SOP_(INT)) of the identified channel outputting from thepolarization transformer PT1 to the target azimuth value (as definedabove, a couple of values mutually orthogonal). Accordingly, themonitoring system MS1 measure the optical power of a polarized portionof solely the identified channel, wherein the polarized portion ispreferably the linearly polarized portion having an azimuth at ±45° tothe defined azimuth. The monitoring system MS1 may generate an outputsignal V₁ indicative of such optical power.

The controller CTRL1 of the first stage 200 is connected to themonitoring system MS1 and it is apt to receive the signal V₁. Thecontroller CTRL1 has an output connected to the birefringent element BE1of the polarization transformer PT1. The controller CTRL1 is apt togenerate an output control signal (labeled φ₁ in FIG. 2), responsive tothe signal V₁, according to a control algorithm. The output controlsignal φ₁ is suitable to be sent to, and to control the polarizationtransformation by, the birefringent element BE1 in order to lock thepolarization azimuth of the identified channel outputting from thepolarization transformer PT1 at the defined azimuth.

The control algorithm is a simple cyclic control algorithm that can beimplemented on a digital PC-based controller (CTRL1), or any othersuitable hardware, firmware, software or combination thereof. Anall-optical processor could also be used for the controller.

Preferably, the control algorithm contains a calculation of an errorvalue, related to the signal V₁, which is related to the displacement ofthe polarization azimuth of the identified channel outputting from thepolarization transformer PT1 from the defined azimuth value. The aim ofthe control algorithm and, more in general, of the control feedback loopis to minimize the above error.

For example, the error may be defined so that it is ideally zero whenthe linearly polarized components of the identified channel (between thetwo stages 200 and 300) at +45° and at −45° to the defined azimuth haveequal optical power.

The minimization of the error is achieved by controlling thepolarization transformation applied by the birefringent element BE1. Thepolarization transformation applied by the birefringent element BE1 istypically varied in a continuous or quasi-continuous manner, with adiscretization that follows from the stepwise incremental nature of thecomputer-implemented control scheme. It is convenient that the steps inthe polarization transformation have a constant absolute value, althoughnon-constant steps, for example dependent on the absolute value of thepolarization transformation, could be used. In general, the smaller thestep, the better the stabilization (smaller uniformity error), but atrade-off with the stabilization speed need to be considered.

At each control period or step the signal control φ₁ of BE1 may bechanged so that the respective polarization transformation changes by aconstant quantity. At each step the control algorithm calculates theerror: if the error at the current step becomes larger than the error atthe previous step, then the sign of the polarization transformationvariation is changed, else the sign is not changed.

The elliptical SOP with fixed axes (SOP_(INT)), obtained as output ofthe first stage 200, is transformed by the second stage 300 into a fixedlinear SOP with optical power independent from the input SOP. In detail,the polarization multiplexed optical radiation outputting the firststage 200 traverses the polarization transformer PT2. The polarizationtransformer PT2 gives to the optical radiation a further controllablepolarization transformation, such that the SOP of the identified channelis transformed from SOP_(INT) to SOP_(OUT), outputting from thepolarization transformer PT2, wherein SOP_(OUT) is a defined SOP(defined azimuth and defined ellipticity). For sake of clarity, thedefined azimuth of the defined output SOP may be different from thepredefined azimuth described above.

The feedback control loop of the second stage 300 of FIG. 2 is designedto lock the SOP (SOP_(OUT)) of the identified channel outputting fromthe polarization transformer PT2 to the target SOP in a way similar tothe feed-back control loop of the first stage 200. To this purpose, themonitoring system MS2 measure the optical power of a polarized portionof solely the identified channel, wherein the polarized portion ispreferably the linearly polarized portion parallel or perpendicular tothe defined SOP. The monitor system MS2 generates respective outputsignal V₃.

The controller CTRL2 of the second stage 300 is connected to the monitorsystem MS2 and it is apt to receive the signal V₃. The controller CTRL2has an output connected to the birefringent element BE2 of thepolarization transformer PT2. The controller CTRL2 is apt to generate anoutput control signal (labeled φ₂ in FIG. 2), responsive to the signalV₃, according to a control algorithm similar to that described withreference to the first stage 200. The output control signal φ₂ issuitable to be sent to, and to control the polarization transformationby, the birefringent element BE2 in order to lock the state ofpolarization of the identified channel outputting from the polarizationtransformer PT2 at a defined SOP.

The controller CTRL2 may execute the same control algorithm as the firststage 200, the only difference being that the error is correlated to V₃.The aim of the feed-back is to minimize or maximize (depending on theazimuth orientation of the fixed polarizer P2) this error.

The fact that the first stage 200 is controlled independently of thesecond stage 300 is highly advantageous, since the provision of twostages does not lead to any additional complexity to the control, sinceno time synchronization between the first and second respectivecontrollers CTRL1 and CTRL2 is required.

Separate controllers CTRL1 and CTRL2 are shown for the first 200 andsecond stage 300 of FIG. 2, consistent with the functional independenceof the control algorithms of the two stages from one another. However,it will be understood that the two controllers could be embodied in asingle hardware, firmware or software unit.

Therefore, the device 100 may comprise a single controller apt tocontrol, responsively to the optical power of the polarized portion ofthe identified channel downstream the polarization transformer PT1, thevariable polarization transformation provided by the polarizationtransformer PT1 so as to maintain the polarization azimuth of theidentified channel downstream the first polarization transformer PT1 atthe predefined azimuth, and is further apt to control, responsively tothe optical power of the polarized portion of the identified channeloutputting from the polarization transformer PT2, the variablepolarization transformation provided by the polarization transformer PT2so as to maintain the state of polarization of the identified channeldownstream the second polarization transformer PT2 at a predefined stateof polarization.

FIG. 2 a shows a possible alternative configuration of the first stage200 of the polarization stabilizer device 100 which is suitable to beused in combination with a superimposed intensity modulation as pilotsignal of the identified optical channel. The alternative configurationof the first stage 200 shown in FIG. 2 a essentially differs from theconfiguration of the first stage 200 shown in FIG. 2 in the monitoringsystem MS1. The devices of stage 200 of FIG. 2 a that are identical tothe devices of stage 200 of FIG. 2 will be indicated with the samereference numeral.

A polarization insensitive beam-splitter BS′ (e.g. with a 90/10 splitratio) may be arranged in the beam path ‘x’ and it is apt to extract asmall fraction (e.g. 10% in this example, or 1%) of the input opticalbeam. The extracted fraction of the input optical beam is directed to aphotodiode PD′ which is apt to measure the power of the extractedfraction. A demodulator DM′ is connected to the output of photodiodePD′.

The beam splitter BS′ shown in FIG. 2 a is located upstream thepolarization transformer PT1, but possible variations would be toarrange the polarization insensitive beam splitter BS′ along the beampath ‘x’ either between the polarization transformer PT1 and the beamsplitter BS1 or downstream the beam splitter BS1. Alternatively, thebeam splitter BS′ can be also located between the beam splitter BS1 andthe polarizer P1.

As shown in FIG. 2 a, a fixed linear polarizer P1 is apt to receive theoptical radiation extracted by the beam splitter BS1. The azimuth of thelinear polarizer P1 may be oriented either at +45° or −45° to thepredefined azimuth (couple of angular values). A photodiode PD1, withits associated demodulator DM1, is optically connected to P1 so that itis apt to measure the power of the polarized component transmitted byP1.

The principle of operation of the first stage 200 of FIG. 2 a is similarto the one exemplarily described for the first stage 200 of FIG. 2. Itis provided a monitoring system MS1 comprising elements (e.g. BS1, P1,PD1, DM1) having the function of extracting a polarized portion (e.g. at+45° or −45° to the defined azimuth) of the optical radiation outputtingfrom the polarization transformer PT1 and generating a signal V₁responsive to the optical power of the extracted polarized portion ofsolely the identified channel, via a demodulation operation performed,e.g., by a demodulator DM1. The detecting system of the first stage 200of FIG. 2 a further comprises elements (e.g. BS′, PD′, DM′) having thefunction of extracting a portion of the optical radiation along the beampath ‘x’ and generating a signal V′ responsive to the pilot signal andindicative of the optical power of solely the identified channelpropagating along the beam path ‘x’.

A controller CTRL1 generates an error value by comparing the opticalpower of the extracted polarized portion of the identified channel(represented by V₁) with a value which is the expected value for thispolarized component when the identified channel outputting from thepolarization transformer PT2 has a polarization azimuth at a definedvalue. Such expected value is calculated based on the signal V′. Forexample, the error value may be defined as ε=|V′−αV₁|, wherein α servesfor the comparison of the extracted polarized portion (V₁) with anexpected value derived from V′. This error serves, through a cyclicfeedback algorithm similar to the one described above, to control theproper polarization transformation at each control step.

Throughout the following description, it will be exemplarily assumedthat the identified channel is identified by a pilot tone, that is tosay a superimposed sinusoidal amplitude modulation, preferably havinglow amplitude and low frequency.

A first embodiment of the polarization stabilizer device of FIG. 2 willnow be described with reference to FIG. 3. The same reference numeralswill be used for elements in FIG. 3 identical to corresponding elementsin FIG. 2. This embodiment is endless and has no intrinsic loss. Inother words, in perfect lossless operation of the components of theoptical device 100, the polarization stabilized output optical radiationcan potentially have up to the full power of the input opticalradiation.

The device 100 of FIG. 3 is apt to receive a polarization multiplexedoptical radiation as an input optical radiation having an identifiedchannel comprising a pilot signal with arbitrary state of polarization(labeled SOP_(IN) in the figure). The polarization multiplexed opticalradiation is emitted from the device 100 as an optical radiation havinga stabilized defined SOP of the identified channel (labeled SOP_(OUT))and an optical power not depending on the input SOP. The defined SOP hasa defined azimuth and a defined ellipticity. Without loss of generality,the defined SOP may be the linear vertical SOP having the definedazimuth vertical and the defined ellipticity zero.

The device 100 comprises a first and a second stage 200 and 300.

The polarization multiplexed optical radiation traverses the first stage200 and outputs the first stage 200 with a SOP (labeled SOP_(INT))having the polarization azimuth at ±45′ with respect to the definedoutput azimuth (i.e. (−45°, +45°) having assumed a vertical outputazimuth). The optical radiation then traverses the second stage 300.

The first stage 200 comprises a polarization transformer PT1 comprisinga first and second variable rotators VPR1 and VPR2, which are variablecircularly birefringent elements with controllable phase retardations Φ₁and Φ₂, respectively. A (polarization) rotator can be seen as abirefringent element with circular eigenstates, that is an element thatrotates the azimuth of the SOP. A circularly birefringent element givinga phase retardation Φ between the circular eigenstates causes a rotationof an angle Φ/2 of the polarization azimuth. The first variable rotatorVPR1 has an associated controllable phase retardations Φ₁ which may havea finite range, i.e. it may have an upper limit or a lower limit, orboth. Advantageously, it may assume one out of two retardation valueswhich are integer multiples of π radians and differ by an odd integermultiple of π radians. The second variable rotator VPR2 has anassociated controllable phase retardations Φ₂ which may have a finiterange. Advantageously, it may smoothly vary at least in a range from kπto (k+k′)π radians, wherein k is an integer and k′ is an odd integer.

In a preferred configuration, the variable rotators VPR1 and VPR2 arevariable Faraday rotators, i.e. variable polarization rotators whichmake use of the magneto-optical Faraday effect and wherein the magneticfield applied to a magneto-optical material is varied.

The polarization transformer PT1 also comprises a quarter-wave plate WP1optically interposed between the first and second variable rotators VPR1and VPR2 and having the eigenaxes oriented at ±45 degree with respect tothe defined azimuth. The quarter-wave plate WP1, as well any othercomponent in the present invention, may be replaced by a technicalequivalent, such as a combination of birefringent elements performingthe same function, without exiting from the scope of the presentinvention. In a preferred configuration, the polarization transformerPT1 consists, for what concerns the optical birefringent elements, onlyof the first and second variable rotators VPR1 and VPR2 and thequarter-wave plate WP1 optically interposed therebetween. Such apolarization transformer PT1 is advantageous due to its simplicity andconsequently low insertion loss, high feasibility and high accuracy.

A monitoring system MS1 is provided to the first stage 200 in allsimilar to that described with reference to FIG. 2. Alternatively, themonitoring system MS1 of FIG. 2 a may be used. It comprises apolarization insensitive beam-splitter BS1 to extract a small fractionof the optical radiation outputting from the second variable rotatorVPR2, a polarizing beam splitter PBS having azimuth approximatelyparallel or perpendicular to the defined azimuth, a first and a secondphotodiode PD1 and PD2, and a first and a second demodulator DM1 and DM2apt to generate respective signals V₁ and V₂ responsive of the opticalpower of, respectively, the two polarized components of solely theidentified channel outputting form the PBS.

A controller (e.g. electronic) CTRL1 is connected to the first andsecond demodulator DM1 and DM2 and it is apt to receive the signals V₁and V₂. The controller CTRL1 has first and second outputs connectedrespectively to the first and second rotators VPR1 and VPR2. Thecontroller CTRL1 is apt to generate output control signals (labeled φ₁and φ₂ in FIG. 3), responsive to the signals V₁ and V₂, according to acontrol algorithm described further below. The output control signals φ₁and φ₂ are suitable to be sent to, and to control the phase retardationsΦ₁ and Φ₂ of, the rotators VPR1 and VPR2, respectively.

In particular applications, photodiode PD2 (and consequently demodulatorDM2) may be omitted. In this case, the polarizing beam splitter PBS maybe replaced by a fixed linear polarizer oriented either parallel orperpendicular to the defined azimuth.

The function of the first stage 200 is to transform any input SOP of theidentified channel into an elliptical output SOP (SOP_(INT)) withprincipal axes at ±45 degrees to said defined azimuth.

In operation, the input polarization multiplexed optical radiationtraverses sequentially the first variable rotator VPR1, the quarter-waveplate WP1 and the second variable rotator VPR2. The first variablerotator VPR1 and the second variable rotator VPR2 rotate the azimuth ofthe optical radiation by respectively a first and a second variableangle Φ₁/2 and Φ₂/2, such that, in combination with the fixed action ofthe quarter-wave plate WP1, the SOP of the identified channel outputtingfrom the second variable rotator VPR2 (SOP_(INT)) has an azimuth at ±45degrees with respect to the defined output azimuth.

A feedback control loop is designed to lock the polarization azimuth ofthe SOP (SOP_(INT)) of the identified channel outputting from the secondrotator VPR2 to the target azimuth value equal to ±45 degrees withrespect to the defined azimuth. The polarization insensitive beamsplitter BS1 diverts a portion of the beam out of the main beam path‘x’. The diverted portion of the beam is then received by the polarizingbeam splitter PBS which splits the diverted beam portion into its twoorthogonal polarization components, which are supplied to, and detectedby, the respective photodiodes PD1 and PD2. The demodulators DM1 and DM2act on the signals generated by the photodiodes PD1 and PD2 and theysupply respective signals V₁ and V₂ as input signals to the controllerCTRL1.

The controller CTRL1 executes an algorithm described below and generatesthe two signals φ₁ and φ₂, responsive of signals V₁ and V₂, controllingthe phase retardations Φ₁ and Φ₂ respectively of VPR1 and VPR2. Thealgorithm may contain a calculation of an error value which is relatedto the displacement of the polarization azimuth of the optical radiationoutputting from the second variable rotator VPR2 from the target azimuthvalue. The aim of the control loop is to minimize the above error.

For example, the error may be defined as ε=|V₁−αV₂|, where the parameterα is determined so that the error is ideally zero when the linearlypolarized components of the identified channel parallel andperpendicular to the defined azimuth have equal optical power. Thiscondition is equivalent to the target of an elliptical SOP_(INT) withprincipal axes at ±45 degrees to said defined azimuth. For example,considering the case of the stabilizer device 100 of FIG. 3 having anideal PBS and photodiodes PD1 and PD2 having equal responsivities, thevalue of α may be chosen equal to 1. In general, different devices mayhave different values for the parameter α.

In those applications, described above, wherein photodiode PD2 may beomitted, there is acquired at each control period of the feedback looponly one signal V_(out) responsive of the optical power of a polarizedcomponent of solely the identified channel and the error is defined asε=|V_(out)−V_(ref)|, where V_(ref) is set via the CTRL1 taking intoaccount the input optical power and the behavior of the opticalelements, e.g. their insertion losses.

The minimization of the error is achieved by controlling the phaseretardations Φ₁ and Φ₂ of the two variable rotators VPR1 and VPR2. Thephase retardation Φ₂ applied by the second variable rotator VPR2 isvaried in a continuous or quasi-continuous manner, with a discretizationthat follows from the stepwise incremental nature of thecomputer-implemented control scheme. It is convenient that the steps inthe phase retardation Φ₂ have a constant absolute value θ, referred toas the “step angle θ”, although non-constant step angles, for exampledependent on the absolute value of the phase retardation Φ₂, could beused. For example, θ=π/180 radians.

In general, the smaller the step angle size, the better thestabilization (smaller uniformity error), but a trade-off with thestabilization speed need to be considered. In fact, for a given stepangle size θ, the maximum SOP fluctuation on the Poincaré sphere (seebelow) in the step time of the control loop that can be compensated foris about θ/2.

The retardation Φ₂ of VPR2 is varied by the controller CTRL1 in apredefined range from kπ to (k+k′)π radians, wherein k is an integer andk′ is an odd integer different from zero. Preferably, k′ is equal 1.Such a range may be for example between 0 and π radians or between π and2π or between 2π and 3π.

Whenever the input SOP varies to cause the retardation Φ₂ reach athreshold of the predefined range (e.g. kπ or (k+k′)π), then theretardation Φ₁ of the first variable rotator VPR1 is switched by thecontroller CTRL1 between the values mπ and (m+m′)π radians, wherein m isan integer and m′ is an odd integer different from zero. Preferably m′is equal to 1. For example, m may be equal to 0, 1 or 2. At the sametime the sign of the phase retardation increments on the second variableretarder is reversed. In the normal mode of operation, when theretardation of VPR2 is not at threshold limit, then the retardation ofVPR1 is kept constant at, e.g., 0 or π radians. The switching of theretardation of VPR1 allows to overcome the finite birefringence rangewherein VPR2 is operated and to obtain an endless polarizationstabilization, while avoiding any cumbersome reset procedure. As will beexplained below, the combination of VPR1, WP1 and VPR2 are so that theazimuth value of the output SOP (SOP_(INT)) is not appreciably perturbedduring the switching of rotator VPR1, provided that the input SOPvariation is sufficiently small in the switching time.

At each control period or step the signal control φ₂ of VPR2 may bechanged so that the respective phase retardation Φ₂ changes by aquantity of constant step angle θ. At each step the control algorithmcalculates the error: if the error at the current step becomes largerthan the error at the previous step, then the sign of the retardationvariation is changed, else the sign is not changed. The signal controlΦ₁ of the phase retardation of VPR1 is kept constant as long as Φ₂ isnot a limit of the predefined range, e.g. [0,π]. If the value Φ₂ hasreached a range limit and the sign of the retardation variation wouldlead next step Φ₂ outside of the range, then the value of Φ₂ is notchanged at the successive step, whilst the variation sign is invertedand the value of Φ₁ is commuted between 0 and π.

More precisely the control algorithm may consist of the followingexemplary algorithm steps:

-   1. assignment of the constant α, depending on the behavior of the    optical components;-   2. initialization to zero of the error at the previous step ε₀;-   3. initialization of the Boolean value S that can assume only the    values 0 or 1, corresponding to the commutation state of the first    rotator VPR1;-   4. initialization of the second rotator retardation Φ₂ to the middle    range value, e.g. π/2;-   5. initialization of the variation sign σ of the phase retardation    Φ₂;-   6. initialization of the absolute value θ (step angle) of the    variation of the phase retardation Φ₂;-   7. acquisition of V₁ from the first photodiode;-   8. (optional in case of V_(ref)) acquisition of V₂ from the second    photodiode;-   9. calculation of the current error ε as absolute value of (V₁−αV₂);-   10. if the current error ε is greater than the previous error ε₀    then:    -   10.1. inversion of the variation sign σ,-   11. variation of Φ₂ by a quantity of absolute value θ and sign σ,-   12. if Φ₂ is not between 0 and π then:    -   12.1. inversion of the variation sign σ,    -   12.2. variation of Φ₂ by a quantity of absolute value θ and sign        θ,    -   12.3. negation of the Boolean state S, that means commutation of        the state of the first rotator VPR1;-   13. assignment of the current error ε to the previous error ε₀;-   14. updating of Φ₁ as product between S and π,-   15. output of the first phase retardation Φ₁;-   16. output of the second phase retardation Φ₂;-   17. return to algorithm step 7.

Referring to FIG. 3, the second stage 300 comprises a polarizationtransformer PT2 similar to the polarization transformer PT1 of the firststage 200 described above. Accordingly it comprises first and secondvariable rotators VPR3 and VPR4, for example similar to the variablerotators VPR1 and VPR2 of the first stage 200, and an interposedquarter-wave plate WP2 oriented at ±45 degree with respect to thedefined azimuth. The elements VPR3, WP2 and VPR4 are arranged along themain beam path ‘x’ of the polarization stabilizer 100 so as to receivethe polarization multiplexed optical radiation outputting from thepolarization transformer PT1 of the first stage 200. The fullystabilized SOP of the identified channel outputting from thepolarization stabilizer device 100 is labeled SOP_(OUT).

The monitoring system MS2 is identical to the one exemplarily describedwith reference to FIG. 2. Accordingly, it comprises a polarizationinsensitive beam splitter BS2, a fixed linear polarizer P2, a photodiodePD3, which is apt to generate, via a demodulator DM3, a signal V₃responsive of the optical power of the extracted polarized portion ofsolely the identified channel.

A controller CTRL2 is connected to the demodulator DM3 and has first andsecond outputs connected to the first and second rotators VPR3 and VPR4respectively. The signal V₃ is sent to an input of the electroniccontroller CTRL2 that generates as outputs, responsive to the inputsignal V₃, the control signals φ₃ and φ₄ for setting the rotators VPR3and VPR4 to the appropriate phase retardation values Φ₃ and Φ₄.

The controller CTRL2 is operable to ensure that the third variablerotator VPR3 assumes preferably only two retardation values, e.g. 0 andπ radians, while the fourth variable rotator VPR4 has a retardationstep-wise smoothly varying, preferably in the range from 0 to π radians.

Separate controllers CTRL1 and CTRL2 are shown for the polarizationstabilizer 100, consistent with their functional independence from oneanother. However, it will be understood that the two controllers couldbe embodied in a single hardware, firmware or software unit.

In operation, the elliptical SOP with fixed axes (SOP_(INT)), obtainedas output of the first stage 200, is transformed by the second stage 300into a fixed linear SOP having the defined (vertical) azimuth. Theoperation of the second stage 300 of FIG. 3 is controlled by a feed-backcontrol loop based on the one described above. The controller CTRL2executes a control algorithm similar to the one of the first stage 200,the only difference being that in step 9 the current error is now theabsolute value of V₃. The aim of the feed-back is to minimize ormaximize (depending on the azimuth orientation of the fixed polarizerP2) this error.

It is noted that the use of the Faraday magneto-optic effect in thepolarization stabilizer device 100 allows solving the problem of thecriticality of the orientation of the applied field and of the opticalpropagation direction with respect to the internal structure of thematerial; a problem which is typically present in birefringent elementbased on electro-optic or acusto-optic effects. In fact, in variableFaraday rotator, the rotation of the polarization azimuth is directlyproportional to the component of the variable magnetic field appliedalong the direction of propagation of the optical radiation. Varying thedirection of propagation and/or the direction of the applied magneticfield, the resulting eigenstates (i.e. left and right circularlypolarized) do not change.

FIG. 4 is now referred to explain the principles of operation of theproposed polarization stabilizer device 100 of FIG. 3 in terms of aPoincaré sphere representation.

Referring to FIG. 4, each SOP is represented by a point on the sphere,with longitude 2η and latitude 2ξ. The angle η is the azimuth of themajor axis of the polarization ellipse and the quantity tan ξ is theellipticity with sign plus or minus according to whether the SOP isleft-handed or right-handed. The poles L and R correspond to the left(ξ=45°) and the right (ξ=−45°) circular SOP respectively. The points onthe equator represent linearly polarized light with different azimuthsη. In particular the points H and V correspond to the horizontal (η=0°)and the vertical (η=90°) linear SOP respectively. The points Q and Tcorrespond to the linear SOP with azimuth η=45° and η=−45° respectively.

The action of a fixed polarizer (such as P2 in FIG. 3) is to transmitonly the component of light in a fixed SOP. The transmitted fraction ofthe incident optical power is cos²(φ/2), where φ is the angle at thecenter of the sphere between the representative points of incident andtransmitted SOP.

For a generic birefringent element there are two orthogonal states ofpolarization, said eigenstates, which are not changed by the elementitself. The effect of the propagation through a birefringent element isrepresented on the Poincaré sphere by a rotation of an angle Φ about asuitable axis. The diametrically opposite points corresponding to theorthogonal eigenstates belong to and identify this axis of rotation. Theangle of rotation Φ is equal to the phase retardation or phasedifference introduced by the birefringent elements between theeigenstates. In case of linearly birefringent element, that is anelement with linearly polarized eigenstates, it is possible to definetwo orthogonal eigenaxes respectively as the fixed directions of thelinearly polarized optical field corresponding to the eigenstates. Arotator is represented as a birefringent element having its axis ofrotation on the vertical axis passing through the poles L and R, asshown in FIG. 4 with the top curved arrow near the symbols Φ₁ and Φ₂representing the rotation on the sphere corresponding to the rotatorsVPR1 and VPR2, respectively.

In FIG. 4, an arbitrary input SOP (SOP_(IN)) is first transformed intoSOP_(WP1) by the quarter-wave plate WP1, having its axis of rotationpassing through points T and Q and an associated fixed rotation on thesphere of 90°. Then it is transformed by the second rotator VPR2 into aSOP (SOP_(INT)) represented on the Poincaré sphere by a point belongingto the great circle Γ including the points L and Q, that is anelliptical SOP with major axis oriented at ±45° with respect to the(vertical) defined azimuth. Thus, by suitably controlling the phaseretardation Φ₂ of the second rotator VPR2 in the exemplary range between0 and π radians, any input SOP (SOP_(IN)) is transformed into a SOPbelonging to the great circle δ. In other words, the first stage 200locks the polarization state on a meridian of the sphere, i.e. it locksthe polarization azimuth to a defined value represented by a couple ofvalues mutually orthogonal. It is contemplated that any great circle onthe Poincaré sphere may take the place of the meridian Γ in FIG. 4,being the locus of the SOPs having one of the two polarizationparameters (or a combination thereof) fixed. The second stage 300, bycontrolling the phase retardation Φ₄, moves the SOP from the greatcircle Γ into the output linear SOP with azimuth η=90°, corresponding tothe point V (trajectory SOP_(INT)−SOP_(WP2)−SOP_(OUT)).

For the sake of clarity, in FIG. 4 it is assumed that the first and thethird commuted rotators VPR1, VPR3 do not act on the SOP (Φ₁=0 andΦ₃=0).

The endlessness of the control scheme of the first stage 200 will now beillustrated with reference to FIG. 5. To this purpose, it will beassumed that the representative point of the input SOP moves along theexemplary trajectory on the Poincaré sphere shown in FIG. 5 a. Foursuccessive representative positions of the input SOP (labeled withincremental numbers from 1 to 4) will be considered.

FIGS. 5 b-5 e represent the four corresponding SOP evolutions during thepropagation of the optical radiation through the first stage 200. Thepoints labeled with the subscripts VPR1, WP1 and VPR2 representrespectively the SOP outputted by the switched rotator VPR1, the SOPoutputted by the linear plate WP1 and the SOP transmitted by thesmoothly varied rotator VPR2.

Initially (FIG. 5 b), the point 1 (SOP_(IN)) passes unperturbed theswitched retarder VPR1 (phase retardation Φ₁=0). Then it is transformedinto the point 1 _(WP1) by the action of the quarter-wave plate WP1 andsubsequently into the point 1 _(VPR2) (belonging to Γ) by the action ofthe smoothly varied retarder VPR2 with exemplary phase retardationΦ₂=π/2.

The variation of SOP_(IN) shown in the trajectory from point 1 to point2 in FIG. 5 a, is compensated by progressively decreasing the phaseretardation Φ₂ till to zero when the point SOP_(IN) intercepts the greatcircle including V and Q, i.e. the equator (point 2 in FIG. 5 c, Φ ₁=0,Φ₂=0). In fact, after the action of WP1, the SOP is already on the greatcircle r.

The further variation of SOP_(IN) according to FIG. 5 a cannot becompensated simply by decreasing Φ₂ because it has reached its lowerlimit. Therefore, in order to obtain an endless control, the phaseretardation Φ₁ is commuted to π, while Φ₂ is kept constant (i.e. equalto zero). As illustrated in FIG. 5 d, the polarization azimuth of theinput SOP (point 3) is rotated of π/2 by the first variable rotator VPR1by means of a rotation of π around the vertical axis (i.e. Φ₁=π, Φ₂=0).Now the successive variation of SOP_(IN) is compensated by increasing Φ₂(FIG. 5 e, Φ ₁=π, Φ₂=π/2).

It is important to note that during the commutation of the first phaseretardation Φ₁ the SOP moves always on the equator (trajectory 3-3_(VPR1) in FIG. 5 d), which is subsequently transformed into the greatcircle Γ including L and Q by the quarter-wave plate WP1. Duringcommutation, the subsequent rotator VPR2 is either at 0 or π, i.e. ittransforms the circle Γ in itself. In conclusion, during the commutationof VPR1 the SOP transformed by the first stage 200 remains at the targetpolarization azimuth (module 90°), provided that the input SOP is nearlyconstant during the commutation.

The endless operation of the control procedure of the second stage 300of FIG. 3 is now described with reference to FIGS. 6 a-6 c, under theassumption that the representative point of the incident SOP (SOP_(INT))endlessly moves on the great circle Γ in the direction from point Q topoint L.

FIGS. 6 a-6 c represent the corresponding evolution of the SOPs duringthe propagation through the birefringent elements of the second stage300 of FIG. 3. The points labeled with the subscripts VPR3, WP2 and VPR4represent respectively the SOP outputted by the switched rotator VPR3,the linear plate WP2 and the smoothly varied rotator VPR4. In all casesthe output SOP is the linear state represented by the point V.

Initially (FIG. 6 a) the point 1, representative of the first SOP_(INT),is left unaltered by the third rotator VPR3 (Φ₃=0). Then it istransformed into the point 1 _(WP2) by the action of the quarter-waveplate WP2 and subsequently into the point 1 _(VPR4) by the action of thesmoothly varied rotator with exemplary phase retardation Φ₄=3π/4. Whilethe representative point 1 moves along the great circle Γ, the controlalgorithm reacts by increasing the phase retardation Φ₄ until reachingthe value of π when the point SOP_(INT) reaches the north pole L, thatis to say is left circularly polarized (FIG. 6 b, point 2, Φ₃=0, Φ₄=π).The further variation of SOP_(INT) can not be compensated simply byfurther increasing Φ₄ because it has reached the exemplary upper limitof π. Therefore, in order to obtain an endless control, the phaseretardation Φ₃ is commuted from 0 to π, while Φ₂ is kept constant, i.e.equal to π (after commutation: Φ₃=π, Φ₄=π). As illustrated in FIG. 6 b,since the point 2 (SOP_(INT)) is an eigenstate (L) of the variablerotator, it is not perturbed during the switching of the rotator VPR3.This assures that the commutation does not perturb the output power,provided that the SOP_(INT) is nearly constant during the commutation.Now the further variation of SOP_(INT), as illustrated in FIG. 6 c, iscompensated by decreasing Φ₄ (Φ₃=π, Φ₄=3π/4).

The proposed endless polarization stabilizer 100 of FIG. 3 forpolarization multiplexed system has been experimentally tested. Byvarying the electrical current injected in the variable rotators VPR1,VPR2, VPR3 and VPR4 in the range of about 9÷27 mA, it has been possibleto rotate the polarization azimuth in the range 0°÷90°. The measuredresponse time of the VPRs in switching the polarization azimuth from 0°to 90° and vice versa is about 40 μs. This response time is limited bythe electric circuit of the current driver. The control algorithm hasbeen implemented on a single digital signal processing electroniccontroller (CTRL1, CTRL2). The electrical feed-back signals aregenerated by the photodiodes (PD1, PD2, PD3) with lowpass-bandwidth ofabout 200 kHz, in order not to eliminate the frequency components aroundthe pilot tone frequency. These spectral components are needed by thecontroller for stabilizing the SOP of the channel identified by thepilot tone. The signals then go through respective pilot tonedemodulator (DM1, DM2, DM3) and are acquired by the controller, afteranalog-to-digital conversion. Three identical pilot tone demodulatorsare used in the experimentation, characterized by a 3 dB-bandwidth ofabout 30 kHz around the center frequency given by the pilot tonefrequency f_(PT)=82 kHz. It has been experimentally found that thedemodulator response time is less than 200 μs. Such a response time isinversely proportional to the 3 dB-bandwidth of the demodulator. Thestep time of the digital algorithm implemented on the controller hasbeen chosen equal to 200 μs in order to allows each feed-back signalcoming to the corresponding pilot tone demodulator to stabilize.

At each step of the digital control algorithm the processor computes theerror and generates four control signals. These signals, afterdigital-to-analog conversion, control respectively the current driversthat generate the VPRs input currents.

The effectiveness of the polarization stabilizer 100 of FIG. 3 inpolarization tracking has been first verified by considering a single 10Gb/s intensity-modulated channel with on-off-keying non-return-to-zero(OOK-NRZ) modulation format. To this data modulation a pilot tone issuperimposed as a sinusoidal intensity modulation at the pilot tonefrequency f_(PT)=82 kHz, with modulation index m. More precisely thepilot tone modulator adds to the signal an intensity modulation directlyproportional to [1+m sin(2πf_(PT)t)]. The measured pilot tone amplitudesA_(i)(t) i=1 to 3, are directly proportional to m.

In the experimentation the 10 Gb/s intensity-modulated NRZ signal isdirectly detected by a photoreceiver, with electrical bandwidth of 7.5GHz, placed after an optical preamplifier. In FIG. 7 the bit-error-rate(BER) as function of the input power (measured in dBm) to the opticalpreamplifier is shown for various modulation index m. The curves 20, 30,40, 50 correspond to an automatic polarization tracking driven by apilot tone with modulation indexes m respectively equal to 0.025, 0.05,0.075, 0.10, in presence of an endlessly varying SOP inputting thestabilizing device 100. The results are compared with the reference BERcurve, labeled 10, obtained in correspondence of a constant input SOP,without polarization tracking and without pilot tone. A penalty, at BER10⁻⁹, less than 1 dB is measured in case of polarization tracking andpilot tone with m equal to 0.05. The experimented penalties are in goodagreement with the usual ones suffered by standard all-optical networkswith pilot tones.

The modulation index m should not exceed a threshold value in order notto degrade the transmission quality. It should also not be too low sothat the signals V₁, V₂, V₃ have a sufficiently high signal to noiseratio (electrical noise may be generated by the photodiode, thedemodulator and the controller). From FIG. 7, a good trade-off range isfrom 0.01 to 0.10, ends of range included.

FIG. 8 shows the result of an assessment of the penalty generated by thepilot tone and by the polarization tracking for m=0.05. The curves 10and 30 are the same as in FIG. 7. The curve 60 corresponds to a pilottone with m=0.05, an input SOP constant and no polarization tracking,showing a penalty of about 0.5 dB, at BER 10⁻⁹, with respect to the caseof no polarization tracking and no pilot tone (curve 10). The BER curve30 obtained in correspondence of an endlessly varying SOP and anautomatic polarization tracking driven by a pilot tone with m=0.05 showsa penalty of less than 0.5 dB with respect to the curve 60. FIG. 8 showsthat for m=0.05 the operation of the polarization tracking gives a verysmall penalty in addition to the small penalty due to the pilot tonealone.

A second alternative embodiment of the polarization stabilizer of FIG. 2will now be described with reference to FIG. 9. The same referencenumerals will be used for identical elements.

The device 100 of FIG. 9 is apt to receive a polarization multiplexedoptical radiation as an input optical radiation having an identifiedchannel comprising a pilot signal with arbitrary state of polarization(SOP_(IN)).

The polarization multiplexed optical radiation is emitted from thedevice 100 as an optical radiation having a stabilized defined SOP ofthe identified channel (SOP_(OUT)). Without loss of generality, thedefined SOP is assumed to be the linear vertical SOP having the definedazimuth vertical and the defined ellipticity zero. The device 100comprises a first and a second stage 200 and 300.

The polarization multiplexed optical radiation traverses the first stage200 and outputs the first stage 200 with a SOP of the identified channel(SOP_(INT)) having the polarization azimuth parallel or perpendicularwith respect to the defined output azimuth (i.e. (0°, 90°) havingassumed a vertical output azimuth). The optical radiation then traversesthe second stage 300.

The first polarization transformer PT1 of the first stage 200 comprisesa first and a second variable retarder VR1 and VR2. The secondpolarization transformer PT2 of the second stage 300 comprises a thirdand a fourth variable retarder VR3 and VR4. A variable retarder is abirefringent element having fixed birefringence eigenaxes and variablecontrollable phase retardation. The eigenaxes of the first variableretarder VR1 are oriented at approximately ±45° with respect to theeigenaxes of the second variable retarder VR2. The same is valid for thethird and the fourth variable retarders VR3 and VR4. The eigenaxes ofthe third variable retarder VR3 are oriented approximately parallel (orperpendicular) with respect to the eigenaxes of the second variableretarder VR2, and the eigenaxes of both the variable retarders VR2 andVR3 are parallel (or perpendicular) with respect to the defined outputSOP (vertical linear).

The monitoring systems MS1 and MS2 and the controller CTRL1, CTRL2 ofthe optical device 100 of FIG. 9 have been described above withreference to FIG. 2 (or FIG. 2 a) and FIG. 3.

The principle of operation of the optical device 100 of FIG. 9, as wellas the control algorithms and the endless mechanism, are similar tothose described with reference to FIG. 3. Further details are describedin patent application WO03/014811 cited above.

An third alternative embodiment of the polarization stabilizer of FIG. 2will now be described with reference to FIG. 10.

The device 100 of FIG. 10 is apt to receive a polarization multiplexedoptical radiation as an input optical radiation having an identifiedchannel comprising a pilot signal with arbitrary state of polarization(SOP_(IN)). The polarization multiplexed optical radiation is emittedfrom the device 100 as an optical radiation having a stabilized definedSOP of the identified channel (SOP_(OUT)) and an optical power notdepending on the input SOP. Without loss of generality, the defined SOPmay be the linear vertical SOP having the defined azimuth vertical andthe defined ellipticity zero.

The device 100 comprises a first and a second stage 200 and 300.

The polarization multiplexed optical radiation traverses the first stage200 and outputs the first stage 200 with a SOP (SOP_(INT)) having thepolarization azimuth at ±45° with respect to the defined output azimuth(i.e. (−45°, +45°) having assumed a vertical output azimuth).

The first polarization transformer PT1 of the first stage 200 comprisesa rotating plate RP1 which may be a quarter-wave plate or a half-waveplate. A rotating plate is a linearly birefringent element having fixedphase retardation and birefringence axes with a controllable rotation.

The second polarization transformer PT2 of the second stage 300comprises a fixed quarter-wave plate WP2 oriented with its axes at ±45°to the output SOP (vertical) and a rotating half-wave plate RP2.

The monitoring systems MS1 and MS2 and the controller CTRL1, CTRL2 ofthe optical device 100 of FIG. 10 have been described above withreference to FIG. 2, 3 and FIG. 2 a.

The principle of operation of the optical device 100 of FIG. 10 is basedon the one described with reference to FIG. 3, provided that now theendless operation is provided by the infinite rotation of the plates RP1and RP2, and the control algorithm needs to be suitably adjusted in astraightforward way.

It is noted that the prior art polarization stabilizer devices based ona two stage scheme, such as for example those described in WO03/014811,are not suitable to stabilize the polarization of a polarizationmultiplexed radiation. In fact, the superposition of two orthogonallypolarized optical channels (e.g. having the same optical wavelength)results in an overall SOP which depends on the intensities and therelative phase of the two channels. With reference to the Poincarésphere representation, in case of equal intensity of the twoorthogonally polarized channels, the overall SOP is represented by apoint P lying on the great circle equidistant from the two diametricallyopposite points representative of the channels. The actual position ofthis point P on the great circle depends upon the relative phase and itmoves on the great circle as the relative phase between the twosuperposed channels varies in the range from 0° to 360°. Vice verse, theoverall SOP represented by a point S might be obtained by superposingtwo beams with equal intensities and orthogonal SOPs represented by anycouple of diametrically opposite points belonging to the great circledefined as the maximum circle of points equidistant from S.

In an attempt to use the prior art schemes to stabilize a polarizationmultiplexed radiation, the Applicant has understood that those schemestry to stabilize the overall SOP and not the orthogonal SOPs of eachchannel. In fact, when the overall SOP is stabilized, for example, withreference to FIGS. 3 and 4, in the vertical linear point V, theorthogonal SOPs of the two channels lie on the great circle Γ of FIG. 4and they move along Γ as their relative phase changes.

The Applicant has also understood that identifying and measuring theidentified channel at the output end of solely the second stage wouldnot provide the desired technical effects. In fact, the first stage 200would lock the overall SOP, with exemplary reference to FIGS. 3 and 4,on the meridian Γ while the two SOPs of the channels would rapidlyfluctuate everywhere on the Poincaré sphere. The second stage 300 wouldnot be able to lock the SOP of the identified channel in the point V, asit is designed to transform a generic point on Γ in V and the SOP of theidentified channel does not lie on Γ. Having recognized these problems,the Applicant has understood that a suitable design of the stabilizerdevice 100 of the present invention would have provided the desiredperformances.

It will be appreciated that the polarization stabilizer device 100 ofthe present invention provides an output optical radiation having afixed linear SOP of the identified channel. However, other devices basedon this design could provide any other defined SOP that may be desired.For example, circularly polarized SOP, or elliptically polarized SOP, orlinearly polarized SOP with a time variant rotation of a desired angularvelocity. To generate a fixed elliptical output SOP, instead of a linearoutput SOP, it is sufficient to produce a fixed linear SOP as describedabove and then obtain an elliptical SOP with a half-wave plate followedby a quarter-wave plate, both fixed and suitably oriented. Anotheralternative is to add a rotating half-wave plate to transform a fixedlinear SOP into a rotating linear SOP.

The polarization stabilizer devices 100 of FIG. 3 can also be modifiedto obtain any fixed output linear SOP other than vertical linear SOP bysuitable rotation of the element WP1 and WP2 (rotation of the eigenaxesazimuth) and the elements PBS and P2. This generalized configuration isobtained from the configuration represented in FIG. 4 by a suitablerotation of the Poincaré sphere about the vertical (L-R) axis.

More in general, any rigid rotation of the Poincaré sphere shown in FIG.4 results in a respective configuration of the polarization stabilizerdevice 100 shown in FIG. 3 which is contemplated by the presentinvention. The same reasoning hold for devices 100 of FIGS. 9 and 10.

1-24. (canceled)
 25. A method for stabilizing the state of polarizationof polarization multiplexed optical radiation, said polarizationmultiplexed optical radiation comprising an identified channel which isprovided with a pilot signal, comprising: (a) providing to thepolarization multiplexed optical radiation a first controllablepolarization transformation to generate a first transformed opticalradiation; (b) measuring a first optical power of a first polarizedportion of said identified channel of the first transformed opticalradiation; (c) controlling, responsively to said first optical power,the first controllable polarization transformation so that theidentified channel of the first transformed optical radiation has apredefined polarization azimuth; (d) providing to the first transformedoptical radiation a second controllable polarization transformation togenerate a second transformed optical radiation; (e) measuring a secondoptical power of a second polarized portion of said identified channelof the second transformed optical radiation; and (f) controlling,responsively to said second optical power, the second controllablepolarization transformation so that the identified channel of the secondtransformed optical radiation has a predefined state of polarization.26. The method according to claim 25, wherein said first polarizedportion of said identified channel of the first transformed opticalradiation has a polarization azimuth at ±45° with respect to saidpredefined polarization azimuth.
 27. The method according to claim 25,wherein said second polarized portion of said identified channel of thesecond transformed optical radiation has a state of polarizationparallel or perpendicular to said predefined state of polarization. 28.The method according to claim 25, wherein step (b) comprises measuring amodulation amplitude of said pilot signal.
 29. The method according toclaim 28, wherein step (b) comprises extracting a power fraction fromthe first transformed optical radiation, polarizing said power fractionto generate a polarized power fraction, detecting said polarized powerfraction and pass-band filtering said detected polarized power fractionto obtain said modulation amplitude.
 30. The method according to claim25, wherein step (e) comprises measuring modulation amplitude of saidpilot signal.
 31. The method according to claim 30, wherein step (e)comprises extracting a power fraction from the second transformedoptical radiation, polarizing said power fraction to generate apolarized power fraction, detecting said polarized power fraction andpass-band filtering said detected polarized power fraction to obtainsaid modulation amplitude.
 32. The method according to claim 25, furthercomprising measuring a third optical power of a third polarized portionof said identified channel of the first transformed optical radiation.33. The method according to claim 32, wherein said third polarizedportion of said identified channel of the first transformed opticalradiation has the polarization azimuth orthogonal to the polarizationazimuth of said first polarized portion.
 34. The method according toclaim 25, wherein the first controllable polarization transformation isendlessly varying.
 35. The method according to claim 25, wherein thesecond controllable polarization transformation is endlessly varying.36. A method of demultiplexing polarization multiplexed opticalradiation, comprising the method of stabilizing the state ofpolarization of polarization multiplexed optical radiation, wherein saidpolarization multiplexed optical radiation comprises an identifiedchannel which is provided with a pilot signal, according to claim 25,and further comprising separating the identified channel in the secondtransformed optical radiation from a further channel orthogonallypolarized to the identified channel.
 37. A method of transmitting apolarization multiplexed optical signal comprising: providing a pilotsignal to an optical channel to generate an identified channel;polarization multiplexing the identified channel with a further channelat a first location to generate polarization multiplexed opticalradiation; propagating said polarization multiplexed optical radiationat a second location different from the first location; stabilizing thestate of polarization of the polarization multiplexed optical radiationat the second location according to the method of claim 25, to generatea polarization stabilized optical radiation; separating the identifiedchannel of the polarization stabilized optical radiation from thefurther channel; and detecting at least one of said identified andfurther channel.
 38. A device for stabilizing the state of polarizationof polarization multiplexed optical radiation, said polarizationmultiplexed optical radiation comprising an identified channel which isprovided with a pilot signal, comprising: a first polarizationtransformer comprising a first birefringent element operable to providea first variable polarization transformation to the polarizationmultiplexed optical radiation; a first monitoring system responsive tosaid pilot signal and capable of measuring the optical power of a firstpolarized portion of the identified channel downstream the firstpolarization transformer; a controller capable of controllingresponsively to the optical power of said first polarized portion, saidfirst variable polarization transformation so as to maintain thepolarization azimuth of the identified channel downstream the firstpolarization transformer at a predefined azimuth; a second polarizationtransformer positioned downstream the first polarization transformer andcomprising a second birefringent element operable to provide a secondvariable polarization transformation to the polarization multiplexedoptical radiation; and a second monitoring system responsive to saidpilot signal and capable of measuring the optical power of a secondpolarized portion of the identified channel downstream the secondpolarization transformer, the controller being further capable ofcontrolling, responsively to the optical power of said second polarizedportion, said second variable polarization transformation so as tomaintain the state of polarization of the identified channel downstreamthe second polarization transformer at a defined state of polarization.39. The polarization stabilizing device according to claim 38, whereinthe first monitoring system is further capable of measuring the opticalpower of a further polarized portion of the identified channeldownstream the first polarization transformer, wherein said furtherpolarized portion is orthogonal to the first polarized portion.
 40. Thepolarization stabilizing device according to claim 38, wherein the firstpolarization transformer further comprises a third birefringent elementoperable to provide a third variable polarization transformation to thepolarization multiplexed optical radiation.
 41. The polarizationstabilizing device according to claim 40, wherein the controller isconfigured to switch the third variable polarization transformationbetween first and second values when the first variable polarizationtransformation reaches a predefined threshold value.
 42. Thepolarization stabilizing device according to claim 40, wherein each ofthe first birefringent element and the third birefringent elementcomprises a respective variable rotator and the first polarizationtransformer further comprises a quarter-wave plate optically interposedbetween the first and the third birefringent element.
 43. Thepolarization stabilizing device according to claim 40, wherein thesecond polarization transformer further comprises a fourth birefringentelement operable to provide a fourth variable polarizationtransformation to the polarization multiplexed optical radiation. 44.The polarization stabilizing device according to 43, wherein thecontroller is configured to switch the fourth variable polarizationtransformation between third and fourth values when the second variablepolarization transformation reaches a predefined threshold value. 45.The polarization stabilizing device according to claim 38, wherein thefirst monitoring system is configured to measure a modulation amplitudeof said pilot signal so as to measure said optical power of said firstpolarized portion.
 46. The polarization stabilizing device according toclaim 45, wherein the first monitoring system comprises a splitter forextracting a power portion of said polarization multiplexed opticalradiation, a polarization splitter for extracting a polarized portion ofsaid power portion, a photodiode for generating a signal from saidpolarized portion of said power portion and a demodulator for band-passfiltering said signal to obtain said modulation amplitude of said pilotsignal.
 47. An optical polarization demultiplexer comprising thepolarization stabilizing device of claim 38, and a polarization divisiondemultiplexer located downstream the polarization stabilizing device andoriented parallel or perpendicular to said defined state ofpolarization.
 48. A polarization division multiplexing systemcomprising: a polarization transmitter capable of combining a first anda second optical channel having orthogonal polarization, wherein thefirst channel comprises a pilot signal; a transmission line capable oftransmitting said combined first and second optical channel; and anoptical polarization demultiplexer according to claim 47, opticallycoupled to said transmission line and capable of separating said firstand second optical channel.